Selective Adsorption of Oxygen from Humid Air in a Metal–Organic Framework with Trigonal Pyramidal Copper(I) Sites

Abstract

High Resolution Image Download MS PowerPoint Slide High or enriched-purity O 2 is used in numerous industries and is predominantly produced from the cryogenic distillation of air, an extremely capital- and energy-intensive process. There is significant interest in the development of new approaches for O 2 -selective air separations, including the use of metal–organic frameworks featuring coordinatively unsaturated metal sites that can selectively bind O 2 over N 2 via electron transfer. However, most of these materials exhibit appreciable and/or reversible O 2 uptake only at low temperatures, and their open metal sites are also potential strong binding sites for the water present in air. Here, we study the framework Cu I -MFU-4 l (Cu x Zn 5– x Cl 4– x (btdd) 3; H 2 btdd = bis(1 H -1,2,3-triazolo[4,5- b ],[4′,5′- i ])dibenzo[1,4]dioxin), which binds O 2 reversibly at ambient temperature. We develop an optimized synthesis for the material to access a high density of trigonal pyramidal Cu I sites, and we show that this material reversibly captures O 2 from air at 25 °C, even in the presence of water. When exposed to air up to 100% relative humidity, Cu I -MFU-4 l retains a constant O 2 capacity over the course of repeated cycling under dynamic breakthrough conditions. While this material simultaneously adsorbs N 2, differences in O 2 and N 2 desorption kinetics allow for the isolation of high-purity O 2 (>99%) under relatively mild regeneration conditions. Spectroscopic, magnetic, and computational analyses reveal that O 2 binds to the copper(I) sites to form copper(II)–superoxide moieties that exhibit temperature-dependent side-on and end-on binding modes. Overall, these results suggest that Cu I -MFU-4 l is a promising material for the separation of O 2 from ambient air, even without dehumidification.